*''[[Latrodectus geometricus]]'', the brown widow, grey widow, or [[brown button spider]], Africa, USA, S. America, Australia. It is unclear where this spider originated; however it has been discovered in many warm, [[cosmopolitan distribution|cosmopolitan]] locales.

*''[[Latrodectus geometricus]]'', the brown widow, grey widow, or [[brown button spider]], Africa, USA, S. America, Australia. It is unclear where this spider originated; however it has been discovered in many warm, [[cosmopolitan distribution|cosmopolitan]] locales.

Latrodectus is a genus of spider, in the family Theridiidae, that contains 31 recognized species. The common name widow spiders is sometimes applied to members of the genus due to the supposed habit of the female of eating the male after mating, although the males of most species are not usually eaten after mating, and can even go on to fertilize other females.[1] The black widow spiders are perhaps the best-known members of the genus. The injection of neurotoxic venom latrotoxin from these species is a comparatively dangerous spider bite, resulting in the condition latrodectism, named for the genus. The female black widow's bite is particularly harmful to humans because of its unusually large venom glands.

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Not all adult female black widows exhibit the red hourglass on their abdomen—some may have a pair of red spots or have no marking at all, but any markings that are present are bright red. Adult male black widows are a quarter the size of the female, and are usually gray or brown rather than black and red; while they may sometimes have an hourglass marking on their abdomen, it is usually yellow or white, not red. The bite of a male black widow is not considered dangerous to humans; it is the bite of the adult female black widow from her much larger venom sacs that has given this spider its dangerous reputation. While there is great variation in specifics by species and by gender, any spider exhibiting a red hourglass on the abdomen and having a shiny black body is an adult female black widow.

Spiders of the genus Steatoda (also of the Theridiidae family) are often mistaken for widow spiders, and are known as "false widow spiders"; they are significantly less harmful to humans.

In common with other members of the Theridiidae family, the widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very frequently hangs upside down near the center of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and wrap it in silk. If the spider perceives a threat, it will quickly let itself down to the ground on a safety line of silk. As other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. While there are some more aggressive species, most are not; many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched. Some bites are thought to result from a spider mistaking a finger thrust into its web for its normal prey[citation needed], or in cases where a female is protecting an egg sac, but ordinarily intrusion by any large creature will cause these spiders to flee.

Silk from L. hesperus spiders is reputed to be particularly strong compared with the silk of other spiders.[2][3] However, the results of a study by Blackledge, et al. do not confirm this.[4]

The ultimate strength and other physical properties of Latrodectus hesperus (western black widow) silk were found to be similar to the properties of silk from orb-weaving spiders that had been tested in other studies. The ultimate strength for the three kinds of silk measured in the Blackledge study was about 1000 MPa. The ultimate strength reported in a previous study for Nephila edulis was 1290 MPa ± 160 MPa.[5] The tensile strength of spider silk is comparable to that of steel wire of the same thickness;[6] as silk is about six times less dense than steel,[7] it is correspondingly stronger than the same weight of steel.

The southern black widow, as well as the closely related western and northern species which were previously considered the same species, has a prominent red hourglass figure on the underside of its abdomen.

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Many of the other widow spiders have red patterns on a glossy black or dark background, which serve as a warning. Spiders which are found in multiple regions are listed in their predominant native habitat.